PHP 8.4 deprecations

Problem/Motivation

There is a PHP 8.4 deprecation that is triggered:

> Deprecated: Drupal\rat\v1\RenderArrayBase::attachFeed(): Implicitly marking parameter $title as nullable is deprecated, the explicit nullable type must be used instead in rat/src/v1/RenderArrayBase.php on line 217

Steps to reproduce

Install and use the module on a PHP 8.4 platform.

Proposed resolution

Fix the deprecations.